How can a woman closing in on the start of her second century be so robustly, almost defiantly, healthy, while men and women decades younger are languishing feebly in nursing homes, plagued with failing bodies and failing minds and wishing they hadn’t been so unlucky as to live so long?

For most of human history, a long and healthy life has been shrugged off as a gift from the gods- or maybe the undeserved reward for a lifetime of plain cussedness. But to gerontologists, the vagaries of aging have become the focus of intense scientific research.

It’s pretty obvious even to nonscientists that how you get there depends partly on the genes you are born with and partly on lifestyle – what and how much you eat, where you live and what types of stress and trauma you experience. How much depends on each factor, though, was unknown until Swedish scientists tackled that problem in 1998. They did it by looking at the only set of people who share genes but not lifestyle: identical twins who were separated at birth and reared apart. If genes were most important, you would expect the twins to die about the same age. In fact, they don’t, and the average difference convinced the scientist that only about 20% to 30% of how long we live is genetically determined. The dominant factor is lifestyle.

“ You could have Mercedes-Benz genes,” says Dr. Bradley Willcox, of the Pacific Health Research Institute in Honolulu, “ but if you never change the oil, you are not going to last as long as a Ford Escort that you take good care of. Those who have healthier genes and live healthier lives- those guys really survive for a long time.”

Those unusually clean-living Americans are genetically diverse, but they avoid alcohol, caffeine and tobacco-and they tend to live an average of eight years longer than their countrymen.

All of this is good news, with a Surgeon General’s warning attached: you can’t change your genes, but you can change what you eat and how much you exercise. “The lesson is pretty clear from my point of view in terms of what the average person should be doing.” Says Perls. “I strongly believe that with some changes in health-related behavior, each of us can earn the right to have at least 25 years beyond the age of 60 – years of healthy life at good function. The disappointing news is that it requires work and willpower.”

By Richard Corliss and Michael D.Lemonick